The invention relates to a method for treating naphtha, such as catalytically cracked naphtha, in order to remove acidic impurities, such as mercaptans. In particular, the invention relates to a method for mercaptans having a molecular weight of about C4 (C4H10S=90 g/mole) and higher, such as recombinant mercaptans.
Undesirable acidic species such as mercaptans may be removed from naphtha and other liquid hydrocarbons with conventional aqueous treatment methods. In one conventional method, the naphtha contacts an aqueous treatment solution containing an alkali metal hydroxide. The naphtha contacts the treatment solution, and mercaptans are extracted from the naphtha to the treatment solution where they form mercaptide species. The naphtha and the treatment solution are then separated, and a treated naphtha is conducted away from the process. Intimate contacting between the naphtha and aqueous phase leads to more efficient transfer of the mercaptans from the naphtha to the aqueous phase, particularly for mercaptans having a molecular weight higher than about C4. Such intimate contacting often results in the formation of small discontinuous regions (also referred to as xe2x80x9cdispersionxe2x80x9d) of treatment solution in the naphtha. While the small aqueous regions provide sufficient surface area for efficient mercaptan transfer, they adversely affect the subsequent naphtha separation step and may be undesirably entrained in the treated naphtha.
Efficient contacting may be provided with reduced aqueous phase entrainment by employing contacting methods that employ little or no agitation. One such contacting method employs a mass transfer apparatus comprising substantially continuous elongate fibers mounted in a shroud. The fibers are selected to meet two criteria. The fibers are preferentially wetted by the treatment solution, and consequently present a large surface area to the naphtha without substantial dispersion or the aqueous phase in the naphtha. Even so, the formation of discontinuous regions of aqueous treatment solution is not eliminated, particularly in continuous process.
In another conventional method, the aqueous treatment solution is prepared by forming two aqueous phases. The first aqueous phase contains alkylphenols, such as cresols (in the form of the alkali metal salt), and alkali metal hydroxide, and the second aqueous phase contains alkali metal hydroxide. Upon contacting the hydrocarbon to be treated, mercaptans contained in hydrocarbon are removed from the hydrocarbon to the first phase, which has a lower mass density than the second aqueous phase. Undesirable aqueous phase entrainment is also present in this method, and is made worse when employing higher viscosity treatment solutions containing higher alkali metal hydroxide concentration.
There remains a need, therefore, for new naphtha treatment processes that curtail aqueous treatment solution entrainment in the treated naphtha, and are effective for removing acidic species such as mercaptan, especially high molecular weight and branched mercaptans.
In an embodiment, the invention relates to a continuous method for treating and upgrading a light and heavy naphtha containing mercaptans, particularly mercaptans having a molecular weight higher than about C4 such as recombinant mercaptans, comprising:
(a) contacting in a first contacting region the light naphtha with a first phase of a treatment composition containing water, alkali metal hydroxide, cobalt phthalocyanine sulfonate, and alkylphenols and having at least two phases, wherein
(i) the first phase contains dissolved alkali metal alkylphenylate, dissolved alkali metal hydroxide, water, and dissolved sulfonated cobalt phthalocyanine,
(ii) at least a portion of the alkyl phenylate is derived from alkyl phenols in the heavy naphtha, and
(iii) the second phase contains water and dissolved alkali metal hydroxide;
(b) extracting mercaptan sulfur from the light naphtha to the first phase, the light naphtha having a lower concentration of alkyl phenols than the heavy naphtha;
(c) contacting in a second contacting region the heavy naphtha with the first phase of the treatment composition, wherein,
(i) the heavy naphtha has a higher boiling range than the light naphtha, and
(ii) the heavy naphtha has a concentration of alkylphenols greater than the concentration in the light naphtha,
(d) extracting mercaptan sulfur and alkylphenols from the heavy naphtha to the first phase;
(e) separating an upgraded light naphtha and separating an upgraded heavy naphtha; and
(f) separating mercaptan sulfur from the first phase, and then conducting the extractant to at least one of step (a) for re-use.
In a preferred embodiment, the process involves conducting the first phase containing mercaptan sulfur from at least one of steps (b) and (d) and conducting an oxidizing amount of oxygen to an oxidizing region and oxidizing the mercaptan sulfur to disulfides; and then separating the disulfides from the first phase. Preferably the contacting is conducted under substantially anaerobic conditions, i.e., without adding oxygen in the contacting. Preferably, the extractant in step (f) is conducted to steps (a) and (c) for re-use.
In another embodiment, the invention relates to a method for treating and upgrading a light and heavy naphtha containing mercaptans, particularly mercaptans having a molecular weight higher than about C4 such as recombinant mercaptans, comprising:
(a) contacting in a first region the light naphtha with an extractant composition containing water, dissolved alkali metal hydroxide, dissolved sulfonated cobalt phthalocyanine, and dissolved alkali metal alkylphenylates, wherein
(i) the extractant is substantially immiscible with its analogous aqueous alkali metal hydroxide,
(ii) at least a portion of the alkali metal alkylphenylate in the extractant is derived from alkyl phenols present in the heavy naphtha, and
(iii) the light naphtha is substantially contains a lower concentration of alkyl phenols than the heavy naphtha;
(b) extracting mercaptan sulfur from the light naphtha to the extractant;
(c) contacting in a second contacting region the heavy naphtha with the extractant, wherein,
(i) the heavy naphtha has a higher boiling range than the light naphtha, and
(ii) the heavy naphtha has a concentration of alkylphenols greater than the concentration in the light naphtha,
(d) extracting mercaptan sulfur and alkylphenols from the heavy naphtha to the extractant;
(e) separating an upgraded light naphtha and separating an upgraded heavy naphtha; and
(f) separating mercaptan sulfur from the extractant, and then conducting the extractant to at least step (a) for re-use.
In a preferred embodiment, the process involves conducting the extractant containing mercaptan sulfur from at least one of steps (b) and (d) and conducting an oxidizing amount of oxygen to an oxidizing region and oxidizing the mercaptan sulfur to disulfides; and then separating the disulfides from the first phase. Preferably the contacting is conducted under substantially anaerobic conditions, i.e., without adding oxygen in the contacting. Preferably, the extractant in step (f) is conducted to steps (a) and (c) for re-use.